1. Field of the Invention
This invention relates to an optical probe, and more particularly to an optical probe having a tubular outer envelope and having a function of deflecting light emitted from the peripheral surface thereof in the direction of circumference or the axis of the outer envelope. The present invention also relates to an optical tomography system where such an optical probe is employed.
2. Description of the Related Art
As a conventional method for obtaining tomographic images of measurement targets, such as living tissue, a method that obtains optical tomographic images by OCT (Optical Coherence Tomography) measurement has been proposed (refer to Japanese Unexamined Patent Publication Nos. 6(1994)-165784 and 2003-139688). The OCT measurement is a type of light interference measurement method that utilizes the fact that light interference is detected only when the optical path lengths of divided light beams, that is, a measurement light beam and a reference light beam, match within a range of coherence length of a light source. That is, in this method, a low coherent light beam emitted from a light source is divided into a measuring light beam and a reference light beam, the measuring light beam is irradiated onto a measurement target, and the measurement light beam reflected by the measurement target is led to a multiplexing means. Whereas the reference light beam is led to the multiplexing means after its optical path length is adjusted so that its optical path length equalizes to that of the reflected light from an arbitrary position in the object. Then the measuring light and the reference light is multiplexed by the multiplexing means, and the intensities thereof are detected by the light detector.
In order to obtain a one-dimensional tomographic image, an interference strength waveform according to the reflectance distribution along the same axis as the direction of travel of the measuring light by scanning the optical path length of the measuring light according to the measuring area. That is, a reflected light intensity distribution according to the structure in the direction of depth of the object to be measured can be obtained. Further, when the projecting position of the measuring light applied to the object is one-dimensionally scanned in a direction perpendicular to the optical axis by the use of a deflecting means or a physical means, a two dimensional tomography representing a reflected light intensity distribution can be obtained. Further, when the projecting position of the measuring light is two-dimensionally scanned in directions perpendicular to the optical axis, a three dimensional tomography representing a reflected light intensity distribution can be obtained.
In the above OCT system, a tomographic image is obtained by changing the optical path length of the reference light, thereby changing the measuring position (the depth of measurement) in the object. This technique is generally referred to as “TD-OCT (time domain OCT)”. More specifically, in the optical path length changing mechanism for the reference light disclosed in Japanese Unexamined Patent Publication No. 6 (1994)-165784, an optical system which collects the reference light emitted from the optical fiber on a mirror is provided and the optical path length is adjusted by moving only the mirror in the direction of the optical axis of the reference light. Further, in the optical path length changing mechanism for the reference light disclosed in Japanese Unexamined Patent Publication No. 2003-139688, the reference light emitted from the optical fiber is turned to parallel light by a lens, the reference light in the form of parallel light is collected and caused to enter the optical fiber again by an optical path length adjusting lens, and the optical path length adjusting lens is moved back and forth in the direction of the beam axis of the reference light.
Whereas, as a system for rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed an optical tomography system for obtaining an optical tomographic image by measurement of SD-OCT (spectral domain OCT). In the SD-OCT system, a tomographic image which is one-dimensional in the optical axis is formed without physically scanning the optical path length, by dividing broad band, low coherent light into measuring light and reference light by the use of an interferometer as in the above-described TD-OCT system, substantially equalizing the measuring light and the reference light to cause them to interfere with each other, decomposing the interference light into the optical frequency components, measuring the intensity of the interference light by the optical frequency components by an array type detector and carrying out a Fourier analysis on the obtained spectral interference waveforms by a computer. As in above-described TD-OCT system, a two-dimensional or a three-dimensional tomographic image can be obtained by scanning the projecting position of the measuring light in directions perpendicular to the optical axis.
As another system for rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed an optical tomography system for obtaining an optical tomographic image by measurement of SS-OCT (swept source OCT). The SS-OCT system employs a light frequency tunable laser as a light source, The high coherence laser beam is divided into measuring light and reference light. The measuring light is projected onto the object and the reflected light from the object is led to the multiplexing means. The reference light is led to the multiplexing means after it is made substantially equal to the measuring light in the optical path length to cause the measuring light and the reference light to interfere with each other, and the measuring light and the reference light are multiplexed by the multiplexing means. The intensity of the multiplexed light is detected by an optical detector. The intensity of the interference light is measured by the frequency component by sweeping the frequency of the light frequency tunable laser and a one-dimensional tomography in the optical axis is formed without physically scanning the optical path length by Fourier-transforming the spectral interference waveform thus obtained with a computer. As in above-described TD-OCT system, a two-dimensional or a three-dimensional tomographic image can be obtained by scanning the projecting position of the measuring light in directions perpendicular to the optical axis.
In the optical tomography system of each of the systems described above, a tomographic image along a certain surface of the object is generally obtained and for this purpose, it is necessary to at least one-dimensionally scan the measuring light beam in the object in perpendicular to the optical axis. As a means for effecting such a light scanning, there has been known, as disclosed in Japanese Patent No. 3104984, an optical probe having a tubular outer envelope and having a function of deflecting light emitted from the peripheral surface thereof in the direction of circumference of the outer envelope. More specifically, the optical probe comprises an inserting portion (outer envelope) which is inserted into the sample, a rotatable hollow shaft which is inserted inside the outer envelope, an optical fiber which is passed through the shaft, and a light deflecting element which is connected to the front end portion of the shaft to be rotated together therewith and deflects light radiated from the front endportion of the optical fiber in a direction of circumference of the outer envelope.
Observing an optical tomographic image has been expected to be developed from the digestive organ which has been reported in the past to a finer region such as a bronchus, a ureter, and a blood vessel. From such a viewpoint, it has been required to make thinner the optical probe. However, in the optical probe disclosed in Japanese Patent No. 3104984, it is necessary to have a certain wall thickness to ensure the strength of the shaft and to ensure a space between the shaft and the optical fiber inside thereof, which makes difficult to make thinner the optical probe.
There is a further demand that a deeper region of the object is to be observed. However, to realize this, it is necessary to make a probe as long as several meters, and it is very difficult to pass an optical fiber through the inside of a shaft which is cylindrical and elongated. Further, the optical fiber can be damaged when it is passed through the inside of such a shaft and accordingly, use of such a probe deteriorates the productivity. Further, since such a cylindrical shaft is high in its manufacturing cost, the optical probe is high in cost. when the optical fiber is rigid, it is conceivable not to pass the optical fiber through the shaft described above and to rotate together with a light deflecting element fixed to the tip of the optical fiber. However, in this case, it is necessary to set a space between the optical fiber and the inner peripheral surface of the outer envelope of the probe in order to smoothly rotate the optical fiber and in such a structure, it is not avoidable the front end the end of the optical fiber wobbling. When so, the orbit of the deflected light emitted from the optical probe is disturbed and it is impossible to construct an accurate tomographic image in the sample.